Why make this post on an old arena that most players have already completed and moved on from? Because it took me over 2 years (on and off...mostly off) to get through vMA and a post like this is what did it. Videos were no help as I really need the choreography of the toughest fights and written is easier for me follow than someone wrecking the bosses in a video using skills that were nerfed to Oblivion six months ago.
My Stage 5 post is here:
https://forums.elderscrollsonline.com/en/discussion/514886/vma-stage-5-ice-stamblade-choreography#latest
I can now complete Stage 7 with just a few deaths. This is a stage that players seem to get stuck on and it's mainly because of the random poison AOEs that appear. If you're struggling with Stage 7, you know all about the poison so I'm not going to explain how it works. I assume you know all too well how it works. Anyway, here's how I managed to get through the poison boss on my stamblade. Note that following these steps perfectly can still result in death due to poison if you get unlucky with a plant spawn.
1. Enter the arena and see if there are any plants in a bad spot -- like right in the middle. "Clear" that plant by running over it and outrunning the AOE circle. If no plants need to be cleared, get ready for the boss.
2. Boss will spawn to your left from where you enter. I place my AOE's near me since he will come running. I also hit him with a Poison Injection and activate my Dark Shadow. I also use Ballista here if it's up. This is NOT a health-based level, it's timer based, so you can DPS as hard and fast as you can.
3. Boss will close to melee range. AVOID his attacks as much as possible. I circle while DPSing and roll-dodge through him when I get behind.
4. After some time (30 seconds?) Two caster argonians appear. KILL ONE. Try to kill the one farthest from the boss but just either will work.
5. Go stand near the other one. The boss will start a shriek while the remaining argonian casts a blue spherical shield. Stand in that and keep fighting the boss. If you step out of the sphere during the shriek, you die. Do NOT kill the mage during the shriek or the shield will drop and you die. I do this by accident a lot with Hail and Blade Cloak. If you got lucky, the boss will be in melee range and you can go ham. If not, you have to use ranged attacks.
6. Once the shriek ends, kill the remaining mage.
7. Repeat steps 3-6. I usually need to do the shriek mechanic a total of two times before the boss dies.
8. In between shrieks, though, I usually grab the axe so I can ensure he's dead during or after the second shriek.
CRITICAL NOTE: If your screen turns green, that means one of the caster things spawned in one of the four corners. Go kill that (usually only takes a poison injection and another hit) UNLESS the boss is shrieking. Never leave the shield if the boss is shrieking as the shriek will kill that caster for you.
Where luck comes in: Sometimes you just get unlucky and a poison plant spawns near your shield. If that happens, I chalk up a loss and start that round over. However, you can try to survive by leaving the shield sphere, bashing the boss (interrupting his shriek) and sprinting to a cleansing pool. I've managed to do this once but promptly died soon thereafter.
Last note: The boss also has a lot of hard-hitting ranged attacks. I'll randomly die to one of those while killing a caster or shield argonian or just trying to avoid melee.